U.S. Pat. No. 5,782,103 discloses a control arrangement, wherein an evaporation pressure of the refrigerant is utilized as a feed-forward parameter. More specifically, the arrangement comprises a PID controller comprising a PI-element and a D-element which is connected in series with the PI-element. The PID controller controls an expansion valve, which in turn control the refrigerant flow from a condenser to an evaporator. A sensor is provided for measuring the temperature of the refrigerant at the inlet of the evaporator or the evaporation pressure in the evaporator. Another sensor measures the temperature of the evaporated refrigerant at the outlet of the evaporator, and a subtractor forms the difference between the two temperatures, i.e. the superheat temperature of the refrigerant. The superheat temperature is supplied as an input to the PI-element, whereas the temperature of the refrigerant at the inlet of the evaporator is supplied via a P-element to the D-element.
Further controllers for controlling expansion valve openings, in which a feedback signal representing the superheat, i.e. the temperature difference between the temperature of the refrigerant at the inlet of the evaporator and the temperature of the evaporated refrigerant at the outlet of the evaporator (or at the inlet of the compressor) are known from U.S. Pat. No. 5,749,238, U.S. Pat. No. 6,018,959, U.S. Pat. No. 4,689,968, U.S. Pat. No. 5,809,794, U.S. Pat. No. 4,807,445, U.S. Pat. No. 4,617,804, U.S. Pat. No. 5,157,934, U.S. Pat. No. 5,259,210, U.S. Pat. No. 5,419,146 and U.S. Pat. No. 5,632,154. Various PI-, PID- and fuzzy logic controllers have been suggested.